This invention relates to a continuously hot dipped metallic coated ferritic chromium alloy ferrous base strip and a process to enhance the wetting of the strip surface with commercially pure molten aluminum.
Hot dip aluminum coated steel exhibits a high corrosion resistance to salt and finds various applications in automatic exhaust systems and combustion equipment. In recent years, automotive combustion gases have increased in temperature and become more corrosive. For this reason, there has become a need to increase high temperature oxidation resistance and salt corrosion resistance by replacing aluminum coated low carbon or low alloy steels with aluminum coated chromium alloy steels. For high temperature oxidation and corrosion resistance, at least part of the aluminum coating layer can be diffused into the iron base by the heat during use to form an Fe-Al alloy layer. If uncoated areas are present in the aluminum coating layer, accelerated corrosion leading to perforation of the base metal may result if the Fe-Al alloy is not continuously formed in the base metal.
It is well known to hot dip metallic coat steel strip without a flux by subjecting the strip to a preliminary treatment which provides a clean surface free of oil, dirt and iron oxide which is readily wettable by the coating metal. Two types of preliminary in-line anneal treatments for carbon steel are described in U.S. Pat. No. 2,197,622 issued to T. Sendzimir and U.S. Pat. No. 3,320,085 issued to C. A. Turner, Jr.
The Sendzimir process for preparation of carbon steel strip for hot dip zinc coating involves passing the strip through an oxidizing furnace heated, without atmosphere control, to a temperature of 1600.degree. F. (870.degree. C.). The heated strip is withdrawn from the furnace into air to form a controlled surface oxide. The strip is then introduced into a reducing furnace containing a hydrogen and nitrogen atmosphere wherein the residence time is sufficient to bring the strip to a temperature of at least 1350.degree. F. (732.degree. C.) and to reduce the surface oxide. The strip is then cooled to approximately the temperature of the molten zinc coating bath and led through a snout containing a protective pure hydrogen or hydrogen-nitrogen atmosphere to beneath the surface of the coating bath.
The Turner process, normally referred to as the Selas process, for preparation of carbon steel strip for hot dip metallic coating involves passing the strip through a furnace heated to a temperature of at least 2200.degree. F. (1204.degree. C.). The furnace atmosphere has no free oxygen and at least 3% excess combustibles. The strip remains in the furnace for sufficient time to reach a temperature of at least 800.degree. F. (427.degree. C.) while maintaining a bright clean surface. The strip is then introduced into a reducing furnace section having a hydrogen-nitrogen atmosphere wherein the strip may be further cooled to approximately the molten coating metal bath temperature and led through a snout containing a protective hydrogen-nitrogen atmosphere to beneath the surface of the coating bath.
U.S. Pat. No. 3,925,579 issued to C. Flinchum et al. describes an in-line pretreatment for hot dip aluminum coating low alloy steel strip to enhance wettability by the coating metal. The steel contains one or more of up to 5% chromium, up to 3% aluminum, up to 2% silicon and up to 1% titanium. The strip is heated to a temperature above 1100.degree. F. (593.degree. C.) in an atmosphere oxidizing to iron to form a surface oxide layer, further treated under conditions which reduce the iron oxide whereby the surface layer is reduced to pure iron matrix containing a uniform dispersion of oxides of the alloying elements.
It is well known that hot dip aluminum coatings do not wet cleaned steel surfaces as easily as zinc coatings. U.S. Pat. No. 4,155,235 to Pierson et al. discloses the importance of keeping hydrogen gas away from the entry section of an aluminum coating bath. This patent teaches a cleaned steel must be protected in a nitrogen atmosphere just prior to hot dip aluminum coating to prevent uncoated spots.
The problems associated with non-wetting of aluminum coatings onto ferritic stainless steel are also well known. Hot dip aluminum coatings are poorly adherent to ferritic stainless steel base metals and normally have uncoated or bare spots in the aluminum coating layer. By poor adherence is meant flaking or crazing of the coating during bending of the strip. To overcome the adherence problem, some have proposed heat treating the aluminum coated steel to anchor the coating layer to the base metal. Others lightly reroll the coated stainless steel to bond the aluminum coating. Finally, those concerned about uncoated spots have generally avoided continuous hot dip coating. Rather, batch type hot dip coating or spray coating processes have been used. For example, after a stainless steel article has been fabricated, it is dipped for an extended period of time within an aluminum coating bath to form a very thick coating layer.
No one has proposed a solution for enhancing the wetting of ferritic chromium alloy steels using hot dip aluminum coatings. Without good surface wetting, the aluminum coating layer will not be uniform, free of uncoated areas and strongly ahderent to the steel base metal. We have discovered a coating method for overcoming the wetting problems associated with hot dip aluminum coating of ferritic chromium alloy steel. The wetting is dramatically improved if a cleaned ferritic chromium alloy steel is maintained in a protective hydrogen atmosphere substantially void of nitrogen prior to the entry of the steel into an aluminum coating bath.